Method of coating metal surface with hexavalent chromium compound and polyacrylic acid



United States Patent lWETHOD OF COATING METAL SURFACE WITH HEXAVALENTCHROMIUM COMPOUND AND POLYACRYLIC ACID Ernest P. Bell, Detroit, Mich.,assignor to Parker Rust Proof Company, Detroit, Mich., a corporation ofMichigan No Drawing. Application July 1, 1955 Serial No. 519,620

4 Claims. (Cl. 117-132) The present invention relates generally tometals or articles of metal having a surface coated with aninsolubilizable or insoluble corrosion-resistant and adhesionpromotingmaterial and to solutions and a method for providing such coatings onmetals. More specifically the invention relates to articles of metalhaving insoluble resinous coatings thereon containing dispersedhexavalent chromium and to solutions and a process for providing suchcoatings.

Metal surfaces have been coated with adherent phosphate or oxalatecoatings to protect the metal against corrosion and to improve theadherence of paints and other finishing materials to the metal. In thecopending application of Donald S. Andrade, Serial No. 519,619, filed ofeven date herewith, there is disclosed a new type of protective andadhesion-promoting coatings for metal comprising hexavalent chromium(dispersed or distributed in, dissolved in or reacted with) an insolubleorganic matrix. In the aforementioned copending application this type ofcoating is formed on the metal by wetting the surface with a solution ofa hexavalent chromium compound such as chromic trioxide or chromic acidin an organic vehicle such as tertiary butyl alcohol and heating thewetted surface to form thereon an insoluble coating comprising aninsoluble organic film-forming or matrix-forming material of unknowncomposition containing dispersed, dissolved or reacted hexavalentchromium. The method of the copending application employs an essentiallyorganic solution which gives rise to the danger of flash fires duringprocessing. The organic vehicles disclosed and claimed in the copendingapplication may be termed monomeric in nature since they are convertedby heat from a liquid, non-polymeric or non-resinous condition to aninsoluble, polymeric or heat-converted form.

In accordance with the present invention, bare metal articles areprovided with a surface coating of a similar type having propertiessimilar to those of the abovementioned copending application byemploying an essentially aqueous solution containing hexavalent chromiumand a matrix-generating organic vehicle in an already polymeric formwhich is convertible on the metal surface by drying and/or heating to aninsolubilizable or insolubilized resinous form. The character of theinsoluble coatings produced by the process of the present inventionlikewise is not fully understood but it is known that both the polymericmatrix and the hexavalent chromium are present on the metal surface in aform in which they are not easily removed and/or extracted by aqueousand organic solvent systems, and are not sensitive to a humid atmosphereor to organic solvents, paint thinner, etc. In some cases there is someevidence (i.e. presence of trivalent chromium in coating and formationof a sludge on standing of solution) that at least a portion of thechromium in the coating has reacted wtih the matrix material. In allcases the hexavalent chromium is believed to be more or less uniformlydistributed throughout the coating.

The

coated metal is remarkably resistant to corrosion and paints, plasticfilms, and other finishing materials are tightly adherent thereto.

The solutions of this invention are aqueous in nature and contain ahexavalent chromium compound and a polymeric material which becomesinsoluble on drying and/or heating. The hexavalent chromium compound maybe chromic trioxide or chromic acid or any of its Water-dispersible andwater-soluble derivatives including chromate and dichromate salts.Particularly satisfactory results have been obtained with chrornictrioxide or chromic acid and its ammonium, chromium, cadmium, manganese,strontium and aluminum dichromates. Others which may be employed includethe chromate and dichromate of magnesium and the water-soluble alkalimetal (including ammonium) and alkaline earth metal chromates anddichromates.

The proportion of hexavalent chromium need be only that amount requiredto impart insolubility to the film and corrosion-resistance and/oradhesiveness for paints to the metal. When using polyacrylic acid as thepolymeric vehicle, the optimum relative proportions of chromic trioxide(CrO to resin (or organic vehicle material in the solution) lie,preferably, in the range of about 8:1 to 1:1 by weight. When greaterproportions of hexavalent chromium are employed the coating is difficultto insolubilize and the coating is likely to be tacky upon standing in ahumid atmosphere. If greater proportions of resin are employed thecorrosion resistance of the metal and adhesion of paint and otherfinishing materials thereto are likely to be not quite as good.Specifically, when polyacrylic acid is employed as the polymeric vehiclehexavalent chromium equivalent to at least 0.25 to 0.5% by weight ofchromic trioxide (CrO based on total volume of solution, seems indicatedas necessary to yield insoluble coatings. Entirely satisfactory resultsare usually obtained with these vehicles with from 0.5 to 4% (wt/vol).Greater proportions may be utilized up to limit of the solubility orstability in the solution, although additional benefit is not alwaysobtained with CrO contents above about 6%.

The polymeric vehicle of this invention is polyacrylic acid which isdispersible in water, that is, it is either emulsifiable or soluble inwater and can be dried or baked to an insolubilizable or insoluble formon the metal in the presence of hexavalent chromium. By insolubilizableis meant that the aqueous solution or dispersion of Vehicle and chromiumcompound can be 'dried to a stable, substantially-non-tacky intermediatecondition (though yet appreciably soluble) and which, when more stronglyheated, can be converted to a form substantially insoluble in Water andorganic solvents. By insoluble is meant that the matrix or resin and itsdispersed chromium compounds is not easily loosened, or Washed oii, orextracted from the metal surface by Water or organic solvents.

The proportion of polymeric vehicle is not critical as long assufiicient is present in the aqueous medium to deposit a finite,continuous film or coating on the metal surface. With polyacrylic acidas little as 0.1% by weight based on the weight of water has, in somecases, produced excellent, insoluble coatings. In general, at least 0.5%polymer is required for best corrosion resistance and paint adhesion.The proportion of the vehicle can be increased up to the limit of itssolubility or stability in water, if desired. Usually, however, dilutesolutions containing from about 0.5% to about 4% will be foundsatisfactory under most commercial operating conditions. The optimumratios given above should be employed for best corrosion resistance andpaint adhesion. The viscosity of the solution will vary, of course, withthe proportion and nature of the polymeric vehicle and can be increasedor decreased by the addition of suitable thickeners, thinners andmodifiers, and other additives, if desired.

'The solution applied to the metal surface can either be acidic oralkaline in nature. For example, solutions containing 0.5 to 6% chromictrioxide (as chromic acid) and 0.5 to 4% of polyacrylic acid are quiteacidic in nature yet are stable. Polyacrylic acid can be partially orcompletely neutralized, for example, with ammonia, before or afteradmixture with a chromium compound. Acidic solutions containing chromicacid can be partially neutralized, for example with ammonium hydroxideand various amines, phosphates, etc., to adjust the pH'of the solutionanywhere in the range of pH of l to 10 for greater storage stability. Ingeneral, unneutralized chromic acid solutionson steel produce coatingssuperior to those produced by the more alkaline solutions, and arepreferred. However, in the case of some metals such as aluminum, forexample, alkaline solutions, produce coatings having superior appearanceand better individual properties.

Fixing of the coating on the surface of the metal may be accomplishedsimply by drying the coating on the surface. In most cases a coatingdried at ordinary temperatures (i.e. 200 F. or below) will beappreciably soluble in water yet it has utility since it can be handledand may be painted and baked to insolubility along with the paint orother finishing material. Where a rapidly-reactive polymeric vehicle isemployed, air drying may produce an insoluble film. In most cases,however, additional heatis usually required to convert the oxidizableresinous vehicle to an insoluble state. In such condition thehygroscopic chromic trioxide (CrO is better protected against humidityand the metal may be handled and stored without difliculty. For thesepurposes temperatures above about 200 .F. are usually required. Attemperatures from above 200 F. to about 500 F. films of the solutions ofpolyacrylic acid with chromic acid are rendered insoluble in as littleas 1 to 5 minutes. In no case should the coating be heated so stronglyor for a time sufficient'to appreciably char the organic polymericvehicle. Drying in an air oven or under infrared radiation can beemployed. Induction heating could also be used. The metal requires nopreliminary treatment unless it is corroded, scaled, dirty or oily. Inthe former cases, an acid pickle will remove corrosion, oxide or scaleand in the latter case an alkaline cleaner such as sodium hydroxide,sodium orthophosphate, sodium orthosilicate, and others, or a solventtreatment may be employed to remove dirt and oil. Followingthe cleaningstep the metal should be rinsed to prevent drag-over of acid, alkali orsolvent to the coating solutions.

The aqueous coating solution may be applied by dipping, spraying, rollercoating, flooding or any other method which will provide a uniform filmof solution on the metal surface. The wetted metal surface is then driedand/orbaked to insolubility.

The invention will now be described more specifically with reference tocertain specific examples which are intended as illustrative only.

Example 1 Water solutions containing 25% by weight of polyacrylic acideach containing a polymer of a difierent molecular weight range wereutilized to prepare coating solutions in this example. These commercialsolutions were'known as Acrysol A-l and Acrysol A-3. Cold rolled steelpanels which had been degreased by a solvent vapor treatment and thendry rag wiped were immersed for one minute in a bath containing '25lbs./ 100 gal. of sodium-orthosilicate at '180 200 F., rinsed for 15seconds in-hot water and then dipped into solutions contain- Insolutions containin either 0.5% polyvinylalcohol (PVA) or 2% polyacrylicacid (Acrysol), certain chromate and dichromate salts were utilized asthe source of hexavalent chromium. Control solutions containing chromiumnitrate were employed to-demonstrate the effect of trivalent chromium.The solutions employed were as follows:

Bath No.

Percent Chromium Compound 1.86 A12(Cr207)3 *Oontrol.

Cold rolled steel panels, which had been vapor degreased, rag wiped andcleaned by dipping for 2 minutes in a bath containing 25 lbs/ gal. ofsodium orthosilicate at to 200 F., and then spray rinsed with hot water,were dipped into these solutions for 5 seconds at room temperature. Thewetted panels were then heated for 5 minutes at 400 F. in an air oven.In every case the hexavalent chromium containing solutions provided agood looking insoluble film. The coated panels were then primed andpainted in the usual fashion and compared to phosphate coated controlpanels and several panels made without a vehicle and containing onlychromium'nitrate.

The Acrysol solutions containing the dichromates of chromium, manganese,cadmium, and aluminum gave similar results, the properties of thepainted panels being only slightly poorer than that of the chromic acidconbrol. The coatings obtained from magnesium chromate and magnesiumdichromate were not quite as good as those obtained from the otherdichromate salts. In most cases, however, the dichromate coated panelswere as good or better than the phosphate-coated control panels. Thedichromate solutions were more stable than comparable solutions madewith chromic acid. Chromium dichromate was stable in polyacrylic acid.In both cases 1.98% and 4.86% chromium nitrate was without substantialeffect as regards improving the corrosion-resistance of the bare metalpanels. It was apparent, therefore, that it is the hexavalent chromiumin the coatings, and not the trivalent or lower valence chromium, thatfunctions in the coatings of this invention to protect the metal andimprove paint adhesion.

Example 3 A series of solutions were prepared from polyacrylic acid andchromic acid containing from 0.25 to 2% 'resin and from 1 to 6% chromicacid. Coated steel panels prepared by heating at 300 to 350 F. and at400 F. revealed several interesting facts. Whenever the ratio of resin:chromic acid was greater than about 1:4, i.e. a higher proportion ofresin, coatings were obtained that were not as good in physical tests asthe 1:4 ratio coatings. Likewise, when the proportionof chromic acid-wasincreased beyond 1:4 the coatings required heating for as much as 20minutes at temperatures of about 400 Frto becomeinsoluble. Other-datadisclosed-in the previous examples indicate that with polyvinylalcoholthe optimum resin: CrO ratio also is around 1:4. This would seemto indicate that these resins are not acting solely as a wetting agentor matrix to disperse and bind the chromium in the coating, but thatthere is an interaction between the resinous vehicle and the chromiumduring drying and baking.

Example 4 Coatings from solutions containing about 0.5% polyvinylalcohol or 0.5% polyacrylic acid, as the polymeric vehicle, and about 2%chromic acid were prepared on steel panels by dipping the cleaned panelsin the solution and force drying for several minutes at about 200 F. Thecoatings produced were substantially soluble in water. However, whenpainted in a conventional manner, with primer and enamel coats, and thenbaked to cure the enamel films, the above panels showed salt spraycorrosion resistance and paint adhesion values substantially equivalentto similar painted panels prepared by first baking the bare coatings at350 F. to reach insolubility before application of paint. This wouldseem to indicate that the coatings of this invention can develop fullinsolubility in situ even though covered with paint. Where the metal isnot stored and exposed to a humid atmosphere before painting, therefore,it appears it may not be necessary to first bake the coatings toinsolubility. Baking is desirable, in most instances however, sincechromic acid picks up moisture and stacked panels are apt to sticktogether and damage the coatings.

What is claimed is:

1. A method of coating a metal surface with an insolublecorrosion-resistant and adhesion-promoting material consisting of thesteps of applying to the entire surface a film of an aqueous admixtureconsisting essentially of water, about 0.25 to about 6% by weight of ahexavalent chromium compound and polyacrylic acid and heating the saidfilm at a temperature in the range of about 250 F.400 F. to insolubilizethe same so as to cover the entire metal surface.

2. A method as defined in claim 1 wherein the hexavalent chromiumcompound is chromic acid.

3. A method of coating a metal surface with an insolublecorrosion-resistant and adhesion-promoting material consisting of thesteps of applying to the entire surface a film of an aqueous admixtureconsisting essentially of Water, about 0.25% to about 6% by weight of ahexavalent chromium compound and polyacrylic acid, said hexavalentchromium compound being present in said admixture relative to saidpolyacrylic acid in the ratio between about 8 CrO 1 polyacrylic acid toabout 1 OrO 1 polyacrylic acid by weight, and heating the said film at atemperature in the range of about 250 F.400 F. to insolubilize the sameso as to cover the entire metal surface.

4. A method as defined in claim 3 wherein the hexavalent chromiumcompound is chromic acid.

References Cited in the file of this patent UNITED STATES PATENTS1,981,102 Hagedorn et al. Nov. 20, 1934 2,025,996 Maillet Dec. 31, 19352,148,862 Kern Feb. 28, 1939 2,174,629 Greiner Oct. 3, 1939 2,184,310Meigs et al. Dec. 26, 1939 2,296,070 Thompson et al Sept. 15, 19422,385,800 Douty et a1 Oct. 2, 1945 2,465,295 Strauss Mar. 22, 19492,519,348 Burnell et al. Aug. 22, 1950 2,562,119 Haon July 24, 19512,567,108 Hochberg Sept. 4, 1951 2,692,826 Neugebauer et a1 Oct. 26,1954 2,716,060 Lupo Aug. 23, 1955 FOREIGN PATENTS 978,446 France Nov.29, 1950 OTHER REFERENCES Mattiello: Protective and Decorative Coatings,volume II, pages 271 and 278, John Wiley and Sons, New York, 1942.

1. A METHOD OF COATING A METAL SURFACE WITH AN INSOLUBLECORROSION-RESISTANT AND ADHESION-PROMOTING MATERIAL CONSISTING OF THESTEPS OF APPLYING TO THE ENTIRE SURFACE A FILM OF AN AQUEOUS ADMIXTURECONSISTING ESSENTIALLY OF WATER, ABOUT 0.25% TO ABOUT 6% BY WEIGHT OF AHEXAVALENT CHRONIUM COMPOUND AND POLYACRYLIC ACID AND HEATING THE SAIDFILM AT A TEMPERATURE IN THE RANGE OF ABOUT 250* F.-400* F. TOINSOLUBILIZE THE SAME SO AS TO COVER THE ENTIRE METAL SURFACE.